1. Field of the Invention
This invention relates to temperature sensitive polymer compositions which are used in flexible temperature or heat sensors for use in electric heaters.
2. Description of the Prior Art
Temperature sensitive polymer materials are generally used in the form of a sheet, which is provided between a pair of wound electrodes to form a temperature sensitive device. This type of device is employed as a flexible temperature sensor or thermosensitive heater. The polymer materials ordinarily used for these purposes are specific types of polyamide compositions whose electrostatic capacitance, resistance or impedance varies depending on the temperature. This characteristic variation is utilized for temperature sensing.
The temperature sensitive polymer materials should generally satisfy the following requirements.
(1) Good temperature detection sensitivity. PA1 (2) To rapidly melt when they are heated to temperatures exceeding the melting point and serve as a fuse. PA1 (3) To undergo little change, in relation to time, of electric characteristics by the action of ion polarization during application of a DC electric field. PA1 (4) To suffer little influence of humidity.
Known temperature sensitive polyamide compositions meet requirements (1) and (2) in most instances. As for requirement (3), a number of factors which give great influences on the variation of electric characteristics are considered. For instance, aside from the chemical structure of stabilizer used in the composition, the types of additives and/or the molecular weight of polyamide, the type of temperature sensitive device, i.e. whichever a temperature sensor or a heat sensitive heater, and the application conditions of electric field have great influences. Requirement (4) is not satisfied so far as polyamide compositions are used. This is because even nylon 11 and nylon 12, which have less moisture absorption than the other polyamide resins, suffer great influences by humidity. A number of resin compositions which are improved in moisture absorption are known. For instance, Japanese Patent Publication Nos. 51-30958, 51-41237 and 53-117 disclose temperature sensitive compositions comprising mixtures of polyamide resins and phenolic compounds.
On the other hand, modified polyamide resins have been proposed for use as temperature sensitive materials, including polyamide resins containing N-alkyl-substituted amido units such as described in Japanese Patent Publication No. 57-59603 and Japanese Laid-open Patent Application No. 55-128203. Moreover, Japanese Laid-open Patent Application Nos. 57-206001 and 58-136624 describe polyester-amide resins, and Japanese Laid-open Patent Application Nos. 55-145756 and 55-145757 describe polyether-ester-amide resins.
Polyamide resins containing N-alkyl-substituted amido units are disadvantageous in that when the degree of substitution of the N-alkyl group is increased, the melting point lowers to below 100.degree. C. Since the degree of substitution cannot be thus increased for use as temperature sensitive materials, it is difficult to suppress the variation of electric characteristics, such as electric resistance and/or impedance, caused by moisture absorption.
Resins having ester groups, such as polyester amide resins or polyether-ester-amide resins, have poor resistances to hydrolysis and hot water. In addition, in order to perform the fusing function as required for temperature sensitive polymer materials, the content of the amido groups in the resins should be kept relatively high, so that a great humidity resistance cannot be expected.
Furthermore, these known polyamide resins are flexible and contain relatively large amounts of impurities such as catalysts for polymerization and unreacted monomers, thus being much lower in electric characteristics such as resistance and impedance than nylon 11 and nylon 12. The low electric charcteristics as well as the relative large variation of electric characteristics caused by moisture absorption of the polyamide resins are very disadvantageous when such resins are used singly as temperature sensitive materials.